101
FXUS02 KWBC 040701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT FRI AUG 04 2023
VALID 12Z MON AUG 07 2023 - 12Z FRI AUG 11 2023
...HEAT WAVE TO PERSIST OVER THE SOUTH AND WORK BACK INTO THE
SOUTHWEST...
...EXCESSIVE RAIN/FLASH FLOOD THREAT FOR THE GREAT LAKES INTO THE
NORTHEAST EARLY NEXT WEEK...
..OVERVIEW
EXPECT A STRONG UPPER RIDGE TO REMAIN CENTERED OVER THE
U.S.-MEXICO BORDER, MAINTAINING A SOUTHERN TIER HEAT WAVE WITH
DAILY RECORD HIGH TEMPERATURES POSSIBLE MOST DAYS NEXT WEEK FROM
THE WESTERN GULF COAST REGION THROUGH THE SOUTHERN ROCKIES AND
POSSIBLY INTO ARIZONA. INITIAL UPPER RIDGING ACROSS THE FLORIDA
PENINSULA SHOULD WEAKEN TO THE SOUTH OF AN EASTERN NORTH AMERICA
TROUGH WHOSE EMBEDDED SHORTWAVES (FROM PACIFIC THROUGH NORTHERN
TIER U.S. FLOW ALONG WITH SOME INFLUENCE FROM SOUTHERN CANADA)
SHOULD SUPPORT AT LEAST A COUPLE FRONTAL SYSTEMS THAT MAY FOCUS
AREAS OF LOCALLY HEAVY RAINFALL OVER PORTIONS OF THE
CENTRAL/EASTERN U.S. MEANWHILE A NORTHEASTERN PACIFIC UPPER
TROUGH AND WEAKENING FRONTAL SYSTEM COULD BRING SOME RAINFALL TO
THE PACIFIC NORTHWEST AROUND THE MIDDLE OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MOST GUIDANCE REFLECTS THE GENERAL THEME OF THE LARGE SCALE MEAN
PATTERN, BUT MOST DETAILS INVOLVE SHORTWAVES THAT ARE SMALL ENOUGH
IN SCALE TO HAVE LOW PREDICTABILITY SEVERAL DAYS OUT IN TIME. AT
LEAST FOR NOW THE MODEL/MEAN CONSENSUS SHOWS TWO PRIMARY FEATURES
OF INTEREST. THE FIRST IS A SYSTEM FORECAST TO TRACK NORTHEAST
FROM THE GREAT LAKES, WITH THE SECOND BEING A FRONT THAT DROPS
INTO THE NORTHERN TIER AROUND TUESDAY-WEDNESDAY AND THEN CONTINUES
INTO THE GREAT LAKES/CENTRAL PLAINS.
REGARDING THE INITIAL GREAT LAKES SYSTEM, OPERATIONAL MODELS
SUGGEST IT WILL BE UNUSUALLY STRONG FOR AUGUST. IN FACT THE
DEEPER SIDE OF THE ENVELOPE (12Z ECMWF/00Z CMC) WOULD CHALLENGE
THIS MONTH'S SEA LEVEL PRESSURE RECORDS FOR SOME STATIONS IN
MICHIGAN. THE 18Z GFS STRAYED TO THE SLOW/SOUTH SIDE OF THE
SPREAD, SO GFS INPUT FOR THE FORECAST CAME FROM THE 12Z GFS THAT
COMPARED BETTER TO OTHER MODELS/MEANS. THE NEW 00Z GFS IS GETTING
CLOSER BUT MAY STILL BE SOMEWHAT SOUTH. ON THE OTHER HAND THE 00Z
CMC TRENDS SLOW AFTER MONDAY IN CONTRAST TO THE FAST/NORTH 00Z
UKMET. SHORTWAVE DETAIL ISSUES THEN AFFECT SPECIFICS OF A
POTENTIAL FRONTAL WAVE OVER OR NEAR NEW ENGLAND BY
TUESDAY-WEDNESDAY.
THE TRAILING FRONT DURING MID-LATE WEEK SHOULD HAVE DYNAMIC
SUPPORT FROM ENERGY EMERGING FROM THE NORTHWEST U.S. AS WELL AS AN
UPPER LOW/TROUGH ROTATING THROUGH SOUTHERN CANADA. THIS POTENTIAL
COMBINATION TEMPERS CONFIDENCE BUT AT THE MOMENT THERE IS DECENT
CLUSTERING AMONG MOST MODELS AND ENSEMBLE MEANS. THE MAIN
DISCREPANCY COMES FROM THE 12Z ECMWF WHICH HOLDS ITS SOUTHERN
CANADA UPPER LOW WELL WEST OF OTHER GUIDANCE, THUS CAUSING THE
SURFACE SYSTEM TO LAG CONSENSUS. THE PRIOR 00Z/03 ECMWF WAS
CLOSER TO THE MAJORITY IN PRINCIPLE.
GUIDANCE COMPARISONS FAVORED STARTING WITH A 12Z OPERATIONAL MODEL
BLEND FOR THE FIRST PART OF THE PERIOD AND THEN STEADILY
INCREASING 18Z GEFS/12Z ECENS MEAN INPUT, REACHING 60 PERCENT
TOTAL BY DAY 7 FRIDAY. THIS DOWNPLAYED SEEMINGLY UNFAVORABLE
TRAITS OF THE 18Z GFS EARLY IN THE PERIOD AND 12Z ECMWF DURING THE
LATTER HALF OF THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
THE UNSEASONABLY STRONG SYSTEM FORECAST TO TRACK OUT OF THE
CENTRAL GREAT LAKES DURING THE FIRST HALF OF THE WEEK WILL SPREAD
POTENTIALLY STRONG THUNDERSTORMS/HEAVY RAINFALL FROM THE GREAT
LAKES/OHIO VALLEY INTO THE NORTHEAST. THE DAYS 4-5 EXCESSIVE
RAINFALL OUTLOOKS COVERING THE MONDAY THROUGH TUESDAY NIGHT PERIOD
DEPICT A MARGINAL RISK AREA CORRESPONDING TO THIS RAINFALL THREAT.
CONTINUE TO MONITOR FORECASTS AS SHORTER-RANGE GUIDANCE MAY
EVENTUALLY PROVIDE ENOUGH REFINEMENT TO SUPPORT A SLIGHT RISK
AREA, ESPECIALLY OVER AREAS WITH WET GROUND CONDITIONS. GENERAL
PROGRESSION OF THIS SYSTEM MAY BE ONE MITIGATING FACTOR THAT COULD
KEEP RAINFALL TOTALS FROM BECOMING TOO EXTREME. BY DAY 5 TUESDAY
THE TRAILING FRONT MAY TAKE ON A MORE WEST-EAST ORIENTATION AS IT
DROPS INTO THE LOWER MISSISSIPPI VALLEY/SOUTHEAST, WHICH COMBINED
WITH SUFFICIENT MOISTURE/INSTABILITY, COULD LEAD TO AN AXIS OF
HEAVY RAINFALL OVER THIS REGION. GUIDANCE SIGNALS ARE NOT
EXTREMELY WELL CLUSTERED YET THOUGH, MERITING A MARGINAL RISK AREA
AT THIS TIME. FINALLY, THERE IS A MARGINAL RISK AREA PROPOSED FOR
PARTS OF THE CENTRAL PLAINS ON MONDAY WHEN A NORTHWEST-SOUTHEAST
BAND OF CONVECTION MAY DEVELOP.
THE SOUTHERN TIER RAINFALL SHOULD PERSIST INTO MID-LATE WEEK AS
THE FRONT LINGERS OVER THE AREA AND THEN GRADUALLY LIFTS NORTH AS
A WARM FRONT. DURING THE LATTER HALF OF THE WEEK, THE INTERACTION
OF GULF MOISTURE WITH THIS FRONT AS WELL AS FRONT DROPPING SOUTH
FROM THE NORTHERN TIER SHOULD LEAD TO THE BEST SHOWER/THUNDERSTORM
FOCUS EXTENDING FROM THE EASTERN PLAINS INTO THE OHIO/TENNESSEE
VALLEYS WITH SOME HEAVY RAINFALL POSSIBLE. MEANWHILE THE PACIFIC
NORTHWEST SHOULD SEE SOME RAINFALL TUESDAY-WEDNESDAY AND PERHAPS
LINGERING A BIT INTO THURSDAY AS A WEAKENING FRONT APPROACHES.
ORGANIZED RAINFALL OVER THE NORTHERN ROCKIES DURING THE
SHORT-RANGE TIME FRAME (THROUGH SUNDAY) SHOULD GRADUALLY BECOME
LIGHTER AND MORE SCATTERED OVER THE COURSE OF NEXT WEEK.
THE ONGOING HEAT WAVE OVER THE SOUTHERN TIER SHOULD CONTINUE
THROUGH NEXT WEEK, WITH GREATEST EMPHASIS OVER THE SOUTHERN HIGH
PLAINS WHERE HIGHS COULD REACH 10-15F ABOVE NORMAL. TEMPERATURES
MAY REACH OR EXCEED DAILY RECORD HIGHS MOST DAYS NEXT WEEK FROM
THE WESTERN GULF COAST INTO ARIZONA, WITH THE DESERT SOUTHWEST
SEEING HIGHS REACHING THE 110S ONCE AGAIN. THE FLORIDA PENINSULA
COULD ALSO SEE A FEW RECORD HIGHS EARLY IN THE WEEK BEFORE UPPER
HEIGHTS DECLINE A BIT. RECORD WARM LOWS WILL BE POSSIBLE ACROSS A
BROADER PORTION OF THE SOUTHERN TIER. HIGH HUMIDITY SHOULD LEAD
TO MAXIMUM HEAT INDICES AT LEAST IN THE 110-115F RANGE ALONG AND
NEAR THE WESTERN HALF OF THE GULF COAST. SOME ABOVE AVERAGE
TEMPERATURES WILL ALSO BRIEFLY SPREAD UP THE WEST COAST FOLLOWING
A BIT OF UPPER-RIDGE AMPLIFICATION, WITH HIGHS IN THE CENTRAL
CALIFORNIA VALLEYS CLIMBING AS HIGH AS 105F OR SO NEXT MONDAY.
THE NORTHERN HALF OF THE ROCKIES INTO PARTS OF THE
NORTHERN-CENTRAL PLAINS WILL BE ON THE COOL SIDE NEXT WEEK, WITH
ROCKIES/HIGH PLAINS LOCATIONS SEEING THE BEST POTENTIAL FOR HIGHS
10-15F BELOW NORMAL NEXT MONDAY-TUESDAY.
RAUSCH
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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